A few things to take note of:
1) Aero wheel covers are not legal for track use.
2) The weight at the RIM is what matters when it comes to acceleration. A disc wheel has it's weight distributed more evenly than a spoked wheel. A heavy disc can have a lighter "rim" than a spoked wheel that weighs less.
3) Wheel stiffness also matters, and most discs can me made stiffer than a spoked wheel. A wheel that is light but flexes can waste as much energy as it takes to accelerate a heavier wheel.
4) Aero is more important than weight, even in an event as short as the 500m. 1/3 of the event is spent accelerating, 2/3 is spent battling the wind. Even the 250m the aerodynamics are more important because the whole event (times portion) is spent at speed.